The invention relates to a method for producing core preforms for a foundry, wherein a mixture of an inorganic, refractory foundry sand and a water-glass-based inorganic binder is produced, the mixture is fed into a heated core box, the water contained in the mixture is removed by a physical method, and the core preform is removed from the core box. The invention also relates to a method for making recycled core sand.
In 1962 an article was published in the journal xe2x80x9cFoundry Trade Journalxe2x80x9d describing hardening of sodium silicate-bound foundry sand through dehydration (see xe2x80x9cFoundry Trade Journalxe2x80x9d, May 3, 1962, pp. 537-544).
The test samples consisted of a densified sand sodium silicate mixture. The test samples were dried by applying a vacuum in the range between 0.5 and 3 mm Hg and maintaining the vacuum until approximately 10 to 30% of the moisture was removed from the binder.
The tests relating to the drying process were performed at different temperatures between 100 and 500xc2x0 C. Other tests related to studies to accelerate dehydration by adding CO2 gas.
The article concluded that the addition of CO2 gas is not essential for hardening the core preforms. It was therefore proposed to produce core preforms in practice by a xe2x80x9ccoldxe2x80x9d process employing a vacuum pump with adequate capacity. This would eliminate the need for heating the core box, which was hitherto deemed essential because of the use of thermosetting resins.
It was, however, recognized that it may be difficult to manufacture large quantities of core preforms since large pumps with a high rotation speed would be required to produce an adequate vacuum. Another disadvantage is that between 8 and 16 minutes are required for vacuum drying, and such a long processing time does not lend itself to mass-production of the core preforms.
About 10 years laterxe2x80x94while mostly resin-bound foundry sands were still in usexe2x80x94the journal AFS-Transactions, volume 86, pp. 227-236, published an article about xe2x80x9cEffects of Microwave Heating on Processing of Core Preformsxe2x80x9d by G. S. Cole. Cole describes the microwave treatment of organic binder systems and concludes that the binder fraction can be significantly reduced by using microwaves. This has significant environmental benefits since organic materials must be specially treated during casting and storage and for disposal. Even a microwave drying process, however, requires that precautions be taken to prevent the organic materials contained in the binder from escaping into the exhaust air.
The only non-organic binder material mentioned in Cole""s article is a sodium silicate binder which is either mixed with complex ester hardeners or subjected to a xe2x80x9cchemical drying processxe2x80x9d using xe2x80x9cconventionalxe2x80x9d CO2 gas systems. This treatment may impair the decomposition characteristics of the core after casting, since the used core sand can only be partially regenerated due to lumps formed in the produced glass phases. The use of ester compounds should be avoided due to environmental concerns.
The problems associated with regeneration in the presence of glass phases were addressed in a Handbook entitled xe2x80x9cFoundry Materials and Foundry Processesxe2x80x9d published by the Deutscher Verlag fxc3xcr Grundstoffindustrie (pp. 80-81). FIG. 3.28 on page 83 of the Handbook illustrates the secondary mechanical strength of CO2-hardened water-glass preforms as a function of the casting temperature. Over the past 30 years, this process has become the standard process for conventional and modified binder solutions and produces water-glass-bound foundry materials with the high temperature properties illustrated in FIG. 3.28. With this process, the foundry material has an increased tendency to sinter and may also not be able to adequately break down after casting. In addition, molten phases are produced which during subsequent cooling bind once again with the basic foundry material. The resulting secondary mechanical strength can be reduced by the following measures described on page 84 of the xe2x80x9cHandbookxe2x80x9d:
1. by optimizing the foundry material recipe to reduce alkalinity;
2. by using water-glass solutions with a reduced binder fraction;
3. by adding additives to promote breakdown.
To this date, this problem has not been optimized satisfactorily. To dissolve the outer shells of the binder, which consist of dehydrated sodium silicate or of gel phases formed by chemical reactions as well as of crystallized molten phases and reaction products, the used foundry material must undergo an intensive wet chemical treatment. The ester-hardened foundry materials have partially elastic binder shells, which require a combination of thermal and mechanical separation methods.
In addition, WO-A-86/00033 describes a method for producing core preforms, wherein the foundry sand is mixed with water-glass which forms a binder. Water is removed from the mixture in a core box under the effect of microwaves. The core box is here constructed of a material which is transparent for microwaves, e.g., plastic, rubber or a multi-layer non-metallic material.
It is therefore an object of the present invention to provide a process for the manufacture of core preforms for foundries which does not have the disadvantages described above. The process provides an environmentally friendly and energy-saving method for the mass-production of components in complex shapes, in particular large-size core preforms. The produced components have a sufficiently high flexural strength for handling and a surface which is xe2x80x9csmoothxe2x80x9d in comparison with conventional core sand surfaces. The process also eliminates additives otherwise required for promoting breakdown. When the xe2x80x9cused sandxe2x80x9d is recycled, it is no longer necessary to break up or separate organic materials and to treat the used sand by wet-chemical processes. The process produces a recycling core sand with physical properties that are identical to those of the initially used raw material.